/**
 * Based on "A Practical Analytic Model for Daylight"
 * aka The Preetham Model, the de facto standard analytic skydome model
 * http://www.cs.utah.edu/~shirley/papers/sunsky/sunsky.pdf
 *
 * First implemented by Simon Wallner
 * http://www.simonwallner.at/projects/atmospheric-scattering
 *
 * Improved by Martin Upitis
 * http://blenderartists.org/forum/showthread.php?245954-preethams-sky-impementation-HDR
 *
 * Three.js integration by zz85 http://twitter.com/blurspline
 * Node.js module implementation by Danila Loginov https://loginov.rocks
 */

import * as THREE from "three";

type shaderInterface = {
  uniforms: { [uniform: string]: THREE.IUniform },
  vertexShader: string,
  fragmentShader: string
}

export let SkyShader: shaderInterface = {

  uniforms: {
    luminance: { value: 1 },
    turbidity: { value: 2 },
    rayleigh: { value: 1 },
    mieCoefficient: { value: 0.005 },
    mieDirectionalG: { value: 0.8 },
    sunPosition: { value: new THREE.Vector3() },
    cameraPos: { value: new THREE.Vector3() }
  },

  vertexShader: `
    uniform vec3 sunPosition;
    uniform float rayleigh;
    uniform float turbidity;
    uniform float mieCoefficient;

    varying vec3 vWorldPosition;
    varying vec3 vSunDirection;
    varying float vSunfade;
    varying vec3 vBetaR;
    varying vec3 vBetaM;
    varying float vSunE;

    const vec3 up = vec3( 0.0, 1.0, 0.0 );

    // constants for atmospheric scattering
    const float e = 2.71828182845904523536028747135266249775724709369995957;
    const float pi = 3.141592653589793238462643383279502884197169;

    // wavelength of used primaries, according to preetham
    const vec3 lambda = vec3( 680E-9, 550E-9, 450E-9 );
    // this pre-calcuation replaces older TotalRayleigh(vec3 lambda) function:
    // (8.0 * pow(pi, 3.0) * pow(pow(n, 2.0) - 1.0, 2.0) * (6.0 + 3.0 * pn)) / (3.0 * N * pow(lambda, vec3(4.0)) * (6.0 - 7.0 * pn))
    const vec3 totalRayleigh = vec3( 5.804542996261093E-6, 1.3562911419845635E-5, 3.0265902468824876E-5 );

    // mie stuff
    // K coefficient for the primaries
    const float v = 4.0;
    const vec3 K = vec3( 0.686, 0.678, 0.666 );
    // MieConst = pi * pow( ( 2.0 * pi ) / lambda, vec3( v - 2.0 ) ) * K
    const vec3 MieConst = vec3( 1.8399918514433978E14, 2.7798023919660528E14, 4.0790479543861094E14 );

    // earth shadow hack
    // cutoffAngle = pi / 1.95;
    const float cutoffAngle = 1.6110731556870734;
    const float steepness = 1.5;
    const float EE = 1000.0;

    float sunIntensity( float zenithAngleCos ) {
      zenithAngleCos = clamp( zenithAngleCos, -1.0, 1.0 );
      return EE * max( 0.0, 1.0 - pow( e, -( ( cutoffAngle - acos( zenithAngleCos ) ) / steepness ) ) );
    }

    vec3 totalMie( float T ) {
      float c = ( 0.2 * T ) * 10E-18;
      return 0.434 * c * MieConst;
    }

    void main() {

      vec4 worldPosition = modelMatrix * vec4( position, 1.0 );
      vWorldPosition = worldPosition.xyz;

      gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );

      vSunDirection = normalize( sunPosition );

      vSunE = sunIntensity( dot( vSunDirection, up ) );

      vSunfade = 1.0 - clamp( 1.0 - exp( ( sunPosition.y / 450000.0 ) ), 0.0, 1.0 );

      float rayleighCoefficient = rayleigh - ( 1.0 * ( 1.0 - vSunfade ) );

    // extinction (absorbtion + out scattering)
    // rayleigh coefficients
      vBetaR = totalRayleigh * rayleighCoefficient;

    // mie coefficients
      vBetaM = totalMie( turbidity ) * mieCoefficient;

    }
  `,

  fragmentShader:
    `
    varying vec3 vWorldPosition;
    varying vec3 vSunDirection;
    varying float vSunfade;
    varying vec3 vBetaR;
    varying vec3 vBetaM;
    varying float vSunE;

    uniform float mieDirectionalG;
    uniform vec3 cameraPos;

    // constants for atmospheric scattering
    const float pi = 3.141592653589793238462643383279502884197169;

    const float n = 1.0003; // refractive index of air
    const float N = 2.545E25; // number of molecules per unit volume for air at
    // 288.15K and 1013mb (sea level -45 celsius)

    // optical length at zenith for molecules
    const float rayleighZenithLength = 8.4E3;
    const float mieZenithLength = 1.25E3;
    const vec3 up = vec3( 0.0, 1.0, 0.0 );
    // 66 arc seconds -> degrees, and the cosine of that
    const float sunAngularDiameterCos = 0.999956676946448443553574619906976478926848692873900859324;

    // 3.0 / ( 16.0 * pi )
    const float THREE_OVER_SIXTEENPI = 0.05968310365946075;
    // 1.0 / ( 4.0 * pi )
    const float ONE_OVER_FOURPI = 0.07957747154594767;

    float rayleighPhase( float cosTheta ) {
      return THREE_OVER_SIXTEENPI * ( 1.0 + pow( cosTheta, 2.0 ) );
    }

    float hgPhase( float cosTheta, float g ) {
      float g2 = pow( g, 2.0 );
      float inverse = 1.0 / pow( 1.0 - 2.0 * g * cosTheta + g2, 1.5 );
      return ONE_OVER_FOURPI * ( ( 1.0 - g2 ) * inverse );
    }

    // Filmic ToneMapping http://filmicgames.com/archives/75
    const float A = 0.15;
    const float B = 0.50;
    const float C = 0.10;
    const float D = 0.20;
    const float E = 0.02;
    const float F = 0.30;

    const float whiteScale = 1.0748724675633854; // 1.0 / Uncharted2Tonemap(1000.0)

    vec3 Uncharted2Tonemap( vec3 x ) {
      return ( ( x * ( A * x + C * B ) + D * E ) / ( x * ( A * x + B ) + D * F ) ) - E / F;
    }

    void main() {
    // optical length
    // cutoff angle at 90 to avoid singularity in next formula.
      float zenithAngle = acos( max( 0.0, dot( up, normalize( vWorldPosition - cameraPos ) ) ) );
      float inverse = 1.0 / ( cos( zenithAngle ) + 0.15 * pow( 93.885 - ( ( zenithAngle * 180.0 ) / pi ), -1.253 ) );
      float sR = rayleighZenithLength * inverse;
      float sM = mieZenithLength * inverse;

    // combined extinction factor
      vec3 Fex = exp( -( vBetaR * sR + vBetaM * sM ) );

    // in scattering
      float cosTheta = dot( normalize( vWorldPosition - cameraPos ), vSunDirection );

      float rPhase = rayleighPhase( cosTheta * 0.5 + 0.5 );
      vec3 betaRTheta = vBetaR * rPhase;

      float mPhase = hgPhase( cosTheta, mieDirectionalG );
      vec3 betaMTheta = vBetaM * mPhase;

      vec3 Lin = pow( vSunE * ( ( betaRTheta + betaMTheta ) / ( vBetaR + vBetaM ) ) * ( 1.0 - Fex ), vec3( 1.5 ) );
      Lin *= mix( vec3( 1.0 ), pow( vSunE * ( ( betaRTheta + betaMTheta ) / ( vBetaR + vBetaM ) ) * Fex, vec3( 1.0 / 2.0 ) ), clamp( pow( 1.0 - dot( up, vSunDirection ), 5.0 ), 0.0, 1.0 ) );

    // nightsky
      vec3 direction = normalize( vWorldPosition - cameraPos );
      float theta = acos( direction.y ); // elevation --> y-axis, [-pi/2, pi/2]
      float phi = atan( direction.z, direction.x ); // azimuth --> x-axis [-pi/2, pi/2]
      vec2 uv = vec2( phi, theta ) / vec2( 2.0 * pi, pi ) + vec2( 0.5, 0.0 );
      vec3 L0 = vec3( 0.1 ) * Fex;

    // composition + solar disc
      float sundisk = smoothstep( sunAngularDiameterCos, sunAngularDiameterCos + 0.00002, cosTheta );
      L0 += ( vSunE * 19000.0 * Fex ) * sundisk;

      vec3 texColor = ( Lin + L0 ) * 0.04 + vec3( 0.0, 0.0003, 0.00075 );

      //vec3 curr = Uncharted2Tonemap( ( log2( 2.0 / pow( luminance, 4.0 ) ) ) * texColor );
      // vec3 color = texColor * whiteScale;
      vec3 color = texColor * 0.3;

      vec3 retColor = pow( color, vec3( 1.0 / ( 1.2 + ( 1.2 * vSunfade ) ) ) );

      gl_FragColor = vec4( retColor, 1.0 );

      #if defined( TONE_MAPPING )
        gl_FragColor.rgb = toneMapping( gl_FragColor.rgb );
      #endif
    } `
};